Transistors, and in particular field effect transistors (FETs), are widely used to drive inductive loads. For example, in the automotive industry, Power FETs are used to drive solenoid valves in antilock brake systems (ABS) as well as electronic stabilisation program (ESP) systems.
Inductive loads comprise the characteristics of an electrical conductor that opposes a change in current flow, exhibiting the same effect on current in an electric circuit as inertia does on velocity of a mechanical object.
An inductive load stores energy in a magnetic field. When the current changes direction, or stops, or the magnetic field changes, an electromotive force (emf) is induced back into the conductor through collapsing of the magnetic field. The opposition to the changes in current flow is identified as counter electromotive force (counter emf). The amount of counter emf is in proportion to the rate of change, that is the faster the rate of change of, voltage across the inductive load or current passed through the inductive load, the greater the counter emf.
A consequence of counter emf is that when an inductive load experiences a sudden change in power, it can cause a large counter emf, resulting in a large voltage spike.
It is known for inductive load driver circuits to comprise re-circulation clamps, in order to protect the circuits from the effects of such voltage spikes. FIG. 1 illustrates an example of a known inductive load driver circuit 100.
The circuit 100 comprises an inductive load L1, coupled to one end to a supply voltage Vsup, and to a drain of a transistor M1. The source of the transistor M1 is coupled to ground. The transistor M1 acts as a switch between the inductive load L1 and the ground plane, and effectively drives the inductive load L1.
With such a circuit configuration, when the transistor M1 is switched ‘on’, the voltage at Vox is substantially equal to zero (i.e. connected to ground), creating a potential difference across the inductive load L1, and thereby causing current flow through the inductive load L1. As previously mentioned, inductive loads comprise the characteristics of an electrical conductor that opposes a change in current flow. Consequently, the current flow starts low and increases as the inductive load L1 stores energy in a form of a magnetic field.
When the transistor M1 is switched ‘off’, Vox is no longer grounded, and the inductive load L1 experiences a change in voltage across it. The change in voltage creates a counter emf, caused by a collapse of the magnetic field. The counter emf causes current to continue flowing through the inductive load L1 as the energy previously stored in the magnetic field is released, and is proportional to the rate of change of the voltage across the inductive load. The continued flow of current through the inductive load L1 causes an increase in the voltage at Vox in the form of a voltage spike.
If the voltage at Vox increases too much, it will exceed the breakdown (or avalanche) voltage of the transistor M1.
To protect the transistor M1 from such voltage increases at its drain, the inductive load driver circuit 100 sometimes comprises a re-circulation clamp in a form of a zener diode Z1. The cathode of the zener diode Z1 is coupled to the drain of the transistor M1 and the anode of the zener diode Z1 is coupled to the gate of the transistor M1, as illustrated.
The zener diode Z1 is configured such that, when a voltage increase is created at Vox, due to the transistor M1 being switched ‘off’, the breakdown voltage of the zener diode Z1 is reached before that of the transistor M1. In this way, when the breakdown voltage of the zener diode Z1 is reached, current flows through the zener diode Z1, thereby limiting the voltage at Vox to that of the breakdown voltage of the zener diode Z1.
The resulting current flowing through the zener diode Z1 also flows through a resistor R1 to ground. The flow of current through the resistor R1 creates a potential difference across resistor R1, and thus between the gate and source of transistor M1. Consequently, if the flow of current through the zener diode Z1 is sufficiently great, it will cause the potential difference across the resistor R1, and thereby between the gate and source of the transistor M1, to become sufficient to turn the transistor M1 ‘on’, thereby allowing current to flow from Vox to ground.
In this manner, when a voltage spike of sufficient magnitude is experienced, the zener diode Z1, not only protects the transistor M1 by limiting the voltage between the drain and gate to that of the breakdown voltage of the zener diode Z1, it also switches the transistor M1 back ‘on’, enabling further current to flow to ground through the transistor M1. In this way, the energy from the voltage spike is dissipated through the zener diode Z1, and more significantly through the transistor M1.
As will be appreciated by a skilled artisan, energy is dissipated in a form of heat generated as the current flows through the transistor M1. Consequently, the transistor M1 is limited to being of a minimum size, in order to be capable of dissipating sufficient heat so that the component is not susceptible to overheating.
As will also be appreciated by a skilled artisan, it is desirable to minimise an overall footprint of an integrated circuit (IC) package comprising such an inductive load driver circuit in order to minimise cost. Clearly, the limitation of a minimum size for the transistor M1 subsequently limits the minimisation of the footprint of the IC package. Furthermore, since an IC package in general comprises a plurality of inductive load driver circuits, and therefore a plurality of transistors, the problem aforementioned footprint size is further compounded.
As technology has progressed, switching speeds of inductive driver circuits has been improved significantly. Although such switching speed increases has improved the responsiveness of such drivers circuits, the increase in switching speeds also results in an increase in the rate of change experienced by the inductive load. Consequently, the voltage spike created by the counter emf is increased.
The result is that, although the same amount of energy may be generated, it is generated over a shorter period of time. Consequently, the inductive load driver circuit, and more particularly the transistor, is required to dissipate heat (energy) over a shorter period of time. This in turn requires the transistor (heat dissipation area) to increase in size in order to be able to dissipate the generated heat. The energy to be dissipated is:E=0.5*L*I2  [1]
Consequently, as technology progresses further, rather than the IC packages becoming smaller, they are in fact increasing in size, further compounding the problem of heat dissipation.
Thus, there is a need for an improved circuit integrated circuit and method for dissipating energy released from an inductive load.